A characteristic feature in modern combustion systems is that the engine comprises an electronic control system for bringing the air-fuel-mixture into stoichiometric proportions. On condition that the fuel mixture is stoichiometric or almost stoichiometric a three-way-catalyst, which usually comprises Rh and Pt and/or Pd, is able to reduce the contents of the three kinds of impurities stated above in the exhaust gases, viz. CO, HC and NO.sub.x.
These catalysts are usually constructed in such a way that a carrier body of a monolithic material with a honeycomb structure is coated with a layer, called a washcoat, which consists of:
(1) A support material with a high specific surface area, which is a refractory oxide or a mixture of refractory oxides, e.g. Al.sub.2.sub.O.sub.3, aluminium silicates, minerals from group IV B metals, mullite and clay minerals, preferably Al.sub.2 O.sub.3 in the .gamma. form, .delta. form or .sigma. form, with a specific surface area of 70-100 m.sup.2 / g.
(2) One or more promoters on the surface of the support or in an intimate mixture therewith. Representatives of such promoters are oxides of the metals Sr, Ba and Ca, their aim being to stabilize the surface against sintering at high temperatures. Other additives can be CeO.sub.2 and NiO, which in different ways promote the catalytic properties, e.g. by intensifying the so called water-gas-reaction or by increasing the oxygen storage capacity or by increasing the dispersion of noble metals.
Further one or more noble metals are impregnated on the surface of the support material, either simultaneously with or after the application of the washcoat. There are usually two or three noble metals, which are chosen from the group consisting of Pt, Pd and Rh.
A general description of the above mentioned technology is to be found in "Automobile catalytic converters, K. C. Taylor, Springer-Verlag, Berlin 1984". Moreover there is a number of patents describing different kinds of technology for obtaining catalysts with properties as stated above, e.g. patent application publications or patents SE 446,506, EP 91,814, SE 403,973, GB 2,122,912, EP 56,729 and SE 445,302, which describe the achievement of improved catalyst properties by using promoting additive chemicals in different ways, and further SE 429,927, which describes the achievement of improved catalyst properties by the distribution of the noble metal in a pronounced concentration gradient in the flow direction of the gases in the carrier body.
In these catalysts the noble metal component is the main catalytically active component in the catalytical reaction, and it is the principal aim of the high surface area of the support material to distribute the noble metal so as to obtain a high dispersion of noble metal, which is important for an efficient use of it.
The wording "carrier body" here implies a solid body with a rigid structure of a honeycomb type, preferably a monolith. This body is usually made of a ceramic or metallic material and has as a characterizing feature a great amount of longitudinal channels, through which the exhaust gases can flow. A typical three-way-catalyst can consist of a monolith with a volume of about 2 litres and with a coating of about 20% by weight, in which the noble metal can constitute about 2% by weight of the coating.
The wording "support material" here implies a powdered material with a high specific surface area, e.g. Al.sub.2 O.sub.3 in the .gamma. form, .delta. form or .sigma. form, which separately or in combination with a promoter material and possibly a noble metal is applied on the carrier body, usually in the form of a slurry, which is dried and calcined.
Different automobile manufacturers use different variations of the technology described above. Further so called oxidation catalysts exist, whose purification only concerns carbon monoxide and hydrocarbons (CO activity and HC activity). These catalysts generally do not contain Rh but are similar in other respects. Moreover pellet type catalysts exist, i.e. the above mentioned chemicals are impregnated on pellets of Al.sub.2 O.sub.3 with a high specific surface area, which then replace the monolithic body.
Important properties in an exhaust gas purifying catalyst are:
(1) Low ignition temperature PA1 (2) High activity PA1 (3) High durability
These properties are obtained with the technology described above. A high activity means that the catalyst should be highly active for different air-fuel-mixtures in the engine. The fuel ratio is stated by the so called .lambda. number. If .lambda. is &lt;1 the mixture is rich and if .lambda. is &gt;1 the mixture is lean. A stoichiometric mixture is obtained at .lambda.=1. For three-way-catalysts the CO activity and the HC activity are high for .lambda.&gt;1 and low for .lambda.&lt;1. For NO.sub.x the reverse applies. The main aim for the electronic control system is therefore to maintain the air-fuel-mixture within a narrow ".lambda. window", within which the catalyst can work with a high activity concerning both CO and HC as well as NO.sub.x. The oxygen storage capacity broadens this window, within which the catalyst shows a high activity.